On the first side a first alignment mark is printed by a printer. The first alignment mark consists of 4 squares recorded in such relative positions and orientations that the edges facing away from the other squares are on edges of an imaginary enveloping square. Between the 4 squares there is an open space forming a first cross.
In the prior art method the first image is printed on the first side.
In addition, on the first side a first alignment mark is printed. The first alignment mark consists of 4 squares recorded in such relative positions and orientations that the edges facing away from the other squares are on edges of an imaginary enveloping square. Between the 4 squares there is an open space forming a first cross.
After printing the first image and the first alignment mark, the paper is removed from the printer and fed back in the printer such that the second image can be printed on the second side.
A light source facing the first side of the paper is positioned such that the paper is between the light source and the camera of the vision system. The light source is used to arrange that the first alignment mark is visible through the medium when looking at the second side. A camera facing the second side captures an image of the first alignment mark and projects a second cross on the image. Settings in the printer are then adjusted such that the second cross is between the 4 squares overlapping the first cross. The first cross has wider features than the second cross such that by positioning the medium, the second cross does not overlap with any of the 4 squares.
Then a second alignment mark is printed on the second side, the second alignment mark being an image of the second cross. In addition the second image is printed on the second side. The second alignment mark is printed for later review of the process.
As the human eye is not a calibrated measuring device, it is difficult for an operator to see if the second cross is placed exactly in the middle between the squares in the first cross. In fact even a misalignment in a positive direction may be judged by the operator as a misalignment in a negative direction which would lead to an adjustment increasing the misalignment.
It is an object of the invention to provide a method that improves the accuracy of the alignment method.